Technical Field
The present invention generally relates to access networks and more precisely to Passive Optical Networks (PON).
It finds applications, in particular, in Ethernet Passive Optical Networks (EPON) for point to multi-point communications between a terminal and a plurality of units.
Related Art
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
A PON is a single, shared optical fiber that uses inexpensive optical splitters to divide the single fiber from a Central Office (CO) into separate strands feeding individual subscribers. In such networks, information is carried by laser bursts. PONs are called ‘passive’ because there are no active electronics within the access network, except at subscriber endpoints and at the CO. The single fiber is divided by a passive splitter.
Ethernet Passive Optical Network (EPON) is based on Ethernet standard, unlike other PON technologies, which are based on Asynchronous Transfer Mode (ATM) standard. EPON enables to utilize the economies-of-scale of Ethernet and provides simple and easy-to-manage connectivity to Ethernet-based IP (for ‘Internet Protocol’) equipment, both at the subscriber endpoints and at the CO.
In such networks, the information is exchanged between layers on a per packet basis. Each packet received in a given layer is encoded with a set of encoding parameters specific to this layer. These parameters should be given through network administration means. A Data Link layer is in charge of sharing the physical resource between the subscriber endpoints and the CO. The Data Link layer is composed by two sub-layers namely the Logical Link (LL) layer and the Medium Access Control (MAC) layer. A Physical layer translates logical communications requests from the Data Link layer into hardware-specific operations to affect transmission or reception of electronic signals.
The IEEE 802.3ah EPON specification, which is also called Gigabit EPON (GEPON), defines Multi-Point Control Protocol (MPCP), Point-to-Point Emulation (P2PE) and Physical layer for 1 Gigabit EPON system (meaning that 1 Gigabit of data is transmitted in the network per second). The IEEE 802.3av specification defines extensions (mainly concerning the Physical layer) for 10 Gigabit EPON. At least, the Standard for Service Interoperability in Ethernet Passive Optical Networks (SIEPON) group, also referenced P1904.1, describes system-level requirements needed to ensure service-level, multi-vendor interoperability of EPON equipment. These specifications complement the existing IEEE Standard 802.3 and IEEE Standard 802.1, which ensure the interoperability at the Physical layer and the Data Link layer.
An EPON network usually includes an Optical Line Terminal (OLT), which can be included in the CO, and one or more Optical Network Unit (ONU), which can be in charge of one or more subscribers of the EPON. The number of ONU managed by each OLT is between four and sixty-four in current deployments.
To control a Point-to-Multi-Point (P2MP) fiber network, EPON uses the MPCP. MPCP performs bandwidths assignment, bandwidth polling, auto-discovery and ranging. MPCP is implemented in the MAC layer, introducing the 64-byte Ethernet control messages:                GATE and REPORT messages are used to assign and request bandwidth;        REGISTER message is used to control auto-discovery process.        
The MAC layer is in charge of transmission arbitration that is allowing a given ONU to enable transmission from its peer for a predetermined interval of time (also called transmission window or timeslot). Start and length of the transmission windows dedicated to each ONU are defined by a Dynamic Bandwidth Allocation (DBA) scheduler comprised in the OLT.
GATE message is sent from the OLT to a given ONU and is used to assign one or several transmission window to that ONU.
REPORT message is a feedback mechanism used by an ONU to indicate its buffer occupancy (meaning the length of a queue of waiting data packets to be sent by the ONU) to the OLT, so that the DBA scheduler can define transmission windows that are adapted to the buffer occupancies of the ONUs.
Start and length of transmission windows as well as queue lengths in REPORT messages are expressed in Time Quantum (TQ) defined to be a 16 ns (nanosecond) time interval for example (i.e. time to transmit 2 bytes at a 1 Gigabit per second speed).
For compliancy reason with the IEEE 802 architecture, devices attached to PON implement a Logical Topology Emulation (LTE) function that may emulate either a shared medium or a point-to-point medium. In this later, the objective is to achieve the same physical connectivity as in switched Local Area Network (LAN), where an active splitter is used between the ONUs and the OLT. The OLT (also called legacy OLT) can have a number N of MAC ports (or interfaces), one for each ONU. Each port is identified using a Logical Link Identifier (LLID) that is assigned to each ONU during a registration procedure.
In the downstream direction, meaning from the OLT to the ONUs, Ethernet packets sent by the OLT pass through a 1*N passive splitter and reach each ONU. Each Ethernet packet comprises a frame preamble that stores the LLID of the port to which the packet is intended. Such a functioning is similar to a shared medium network and as Ethernet is perfectly compatible with EPON architecture, as Ethernet is broadcasting by nature. Thus, Ethernet packets are broadcasted by the OLT and selectively extracted by the ONUs by using the LLID that is inserted in the Ethernet frame preamble. Downstream processing in the OLT is very simple since it consists mainly of tagging incoming packets with the right LLID and forwarding them to the corresponding Logical Link.
Therefore, an EPON Data Path (EDP) can be defined as a traffic bearing object within an EPON system, which represents a data or control flow connection. Each service or high level application is mapped to a dedicated EDP, for which is attached a set of Quality of Service (QoS) parameters.
An EDP can be bidirectional unicast or unidirectional (downlink) multicast. Bidirectional unicast EDP can be implemented using two methods:                Service Level Agreement (SLA) using the different queues on a single LLID. Bandwidth parameters are defined via a new configuration message and QoS is guaranteed by a scheduling mechanism implemented in each ONU.        Multiple LLID, in which one queue (i.e. service) is mapped on one LLID using a new configuration message. Consequently, one ONU may register several LLIDs, one for each dedicated service. Bandwidth parameters for each LLID are configured in the OLT only and are allocated by the DBA.        
The Multiple LLID method has several advantages:                upstream resource scheduling (meaning transmission from the ONUs to the OLT) is performed only by the DBA in the OLT. Indeed, as there is only one kind of service mapped over a logical link (i.e. only one traffic queue), there is no need to schedule different queues in the ONU, which simplifies the scheduling mechanism. There is no need to define in the standard how ONU shall deal with priority, type of scheduling algorithm, etc. REPORT message contains only one valid queue with several queue sets optionally;        Virtual LAN tags (outer and inner), which are used to identify services in SLA method, are not considered and it is not required to translate them, thus rendering the method more transparent;        the LLID field is encoded on 15 bits which enables to define 128 LLIDs for each ONU, while considering an OLT managing 128 ONUs, thus rendering the method more scalable;        any legacy OLT is compatible with ONUs that support multiple LLIDs. Indeed, the DBA in the OLT does not deal with ONUs but only with LLIDs. An ONU with many opened LLIDs is considered as a set of independent virtual ONUs from the legacy OLT point of view.        
However, the Multiple-LLIDs method introduces an upstream overhead because of the introduction of additional LLIDs in the EPON architecture.
Indeed, EPON upstream overhead is mainly due to control message overhead and guard band overhead. Main contribution of control message overhead is REPORT messages that are sent by the ONUs to indicate their buffer occupancy. Guard band is a time that is left blank between two upstream bursts in order to switch on/off the laser and perform required calibrations. Other factors that contribute to overhead, as discovery overhead and frame delineation for example, can be considered as negligible.
The REPORT messages can have variable composition: they can contain the buffering occupancy of each queue (or each logical link in the presented case) and different sets can be inserted. However, REPORT message length is fixed to 64 bytes and pads with dummy data. This value being given and considering that one REPORT message is sent by each ONU during one 1 ms (millisecond) cycle, the overhead due to the REPORT message for a 1 Gbit EPON is equal to
                    n                  O          ⁢                                          ⁢          N          ⁢                                          ⁢          U                    ×              (                  64          +          8          +          12                )                    125000000      ⁢                          ×      0.001        ,nONU being the number of ONUs managed by the OLT, 8 bytes being used for the frame preamble and 12 bytes being the inter-frame gap between two adjacent Ethernet frames. For 32 and 128 ONUs, the REPORT message overhead equals respectively 2.15% and 8.6%. For a 10 Gbit EPON, the REPORT message overhead drops under 1% and can thus be considered as negligible.
Referring now to guard band overhead, it comprises:                a laser off time partially overlap by a laser on time dedicated to the next ONU, which is fixed at 512 ns (nanoseconds);        a dead zone of 128 ns between the switching off of the laser of an ONU and the switching on of the laser of the next ONU, to which an opportunity is dedicated;        an Automatic Gain Control (AGC) time set to a discrete value comprised between 96 ns and 400 ns specified by the IEE802.3ah D1.414 specification;        a Clock and Data Recovery (CDR) time set to a discrete value comprised between 96 ns and 400 ns specified by the IEE802.3ah D1.414 specification.        
Considering the worst case, with AGC time and CDR time being equal to 400 ns, guard band time is equal to 1.44 μs (microsecond). Then, considering that each ONU has an opportunity (defined in the GATE messages sent by the OLT) in each 1 ms cycle, the guard band overhead is equal to
                    n                  O          ⁢                                          ⁢          N          ⁢                                          ⁢          U                    ×      1.44        1000    .For 32 and 128 ONUs, the guard band overhead equals respectively 4.6% and 18.4%. The guard band overhead is identical for 1 Gbit and 10 Gbit EPONs.
Table 1 summarises upstream overhead for different numbers of ONUs in a 1 Gbit EPON with a 1 ms DBA cycle, each ONU having an opportunity to send data to the OLT in each cycle.
TABLE 1ControlTotalRemainingNumber ofmessageGuard bandoverheadbytes for userONUsoverhead (%)overhead (%)(%)data1612.33.37548322.124.66.753642644.39.213.516891288.618.42771325617.236.854224
Referring to the results contained in Table 1, it is not reasonable to manage more than 32 ONUs per DBA cycle. Indeed, beyond 32 ONUs per DBA cycle, the overhead exceeds 10% and the transmission window allows transmitting only one 1500-bytes frame since its length expressed in bytes is lower than 2000 bytes.
Thus, there is a need to reduce the overheads due to REPORT messages and guard band in EPON architecture without impacting on the transmission opportunities offered to the ONUs.